Set of glenoid components for a shoulder prosthesis

ABSTRACT

This set comprises glenoid components that each include a body defining, on two of its opposite faces, respectively, a joint surface, intended to cooperate with a humeral head, and a bearing surface bearing against the socket of a shoulder blade. In this set, the glenoid components are provided in several different sizes, respectively defined by the dimensions of the joint surface of their body. At least two glenoid components of which the bearing surfaces respectively have different dimensional geometries are provided so as to allow the surgeon to improve the durability of the mechanical cooperation between the implanted component and the operated socket.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 15/145,210, filed Oct. 21, 2011, now U.S. Pat. No. 10,631,993 which is a divisional of U.S. patent application Ser. No. 13/279,070, now U.S. Pat. No. 9,351,844, filed on Oct. 21, 2011, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/488,102, filed on May 19, 2011, and claims foreign priority to French Patent Application No. FR1058647, filed on Oct. 22, 2010, all of which are incorporated by reference herein in their entireties for all purposes.

TECHNICAL FIELD

Embodiments of the present invention relate to a set of glenoid components for a shoulder prosthesis.

BACKGROUND

Typically, the body of a glenoid component is not very thick, the two opposite primary faces of said body respectively being adapted to cooperate in a ball-and-socket manner with a humeral head, either natural or prosthetic, and to be pressed against the socket of a shoulder blade so as to be immobilized there. To that end, the first aforementioned face includes a joint surface, generally spherical, while the second face includes a bearing surface, which is also spherical or planar, in the central region of which often at least one bone anchoring element protrudes in the socket, such as a keel or pins. Such glenoid components may be provided in different sizes, but the glenoid components of different sizes often have identical bearing surfaces.

Thus, depending in particular on the patient's morphology, the surgeon has the option of choosing the glenoid component of which the size seems best suited to the patient. As a result, the articular performance of the patient's prosthetic shoulder is quite often satisfactory. At the same time, it has, however, been observed that, during use of glenoid components which did not best suit the patient, the implanted glenoid component tends gradually to come unsealed from the socket, through wear or mechanical alteration of the interface between the body and the socket.

SUMMARY

A set of glenoid components for use in a joint according to embodiments of the present invention includes a first glenoid component having a first joint surface and a first bearing surface, and a second glenoid component having a second joint surface and a second bearing surface, wherein each of the first and second joint surfaces is configured to cooperate with a prosthetic or natural bone head, wherein each of the first and second bearing surfaces is configured for placement against a socket of the joint, wherein the first joint surface is substantially the same as the second joint surface, and wherein the first and second bearing surfaces have different dimensional geometries. Both the first and second bearing surfaces may include substantially convex curved surfaces. The first bearing surface may have a first radius of curvature, and the second bearing surface may have a second radius of curvature that is greater than the first radius of curvature. The second radius of curvature is at least 4 mm greater, or 8 mm greater, than the first radius of curvature, according to embodiments of the present invention.

According to some embodiments of the present invention, the first and second glenoid components are of a first size, and the set further includes a third glenoid component having a third joint surface and a third bearing surface, a fourth glenoid component having a fourth joint surface and a fourth bearing surface, wherein each of the third and fourth joint surfaces is also configured to cooperate with the prosthetic or natural bone head, wherein each of the third and fourth bearing surfaces is also configured for placement against the socket of the joint, wherein the third and fourth glenoid components are of a second size larger than the first size, wherein the third joint surface is substantially the same as the fourth joint surface, and wherein the third and fourth bearing surfaces have different dimensional geometries. The first bearing surface may have a first radius of curvature, the second bearing surface may have a second radius of curvature that is greater than the first radius of curvature, the third bearing surface may have a third radius of curvature, and the fourth bearing surface may have a fourth radius of curvature that is greater than the third radius of curvature, according to embodiments of the present invention. In some cases, the first radius of curvature may be the same as the third radius of curvature. In such cases, the second radius of curvature may be the same as the fourth radius of curvature. In other cases, the first radius of curvature may be smaller than the third radius of curvature, and/or the second radius of curvature may be smaller than the fourth radius of curvature.

According to some embodiments of the present invention, the first and third bearing surfaces have identical dimensional geometries. According to other embodiments of the present invention, the different dimensional geometries are two different geometries selected from the group including a planar surface, a convex curved surface, a concave curved surface, and a terraced surface. At least one of the first and second bearing surfaces may include a protruding anchor element configured for engagement with the socket of the joint.

According to some embodiments of the present invention, the first glenoid component includes a first top face and a first bottom face, the first top face including the first joint surface, the first bottom face including the first bearing surface, wherein the first bearing surface occupies most or all of the first bottom face other than an anchor element, if any. According to some embodiments of the present invention, the joint is a shoulder joint, the socket is a shoulder blade socket, and the prosthetic or natural bone head is a prosthetic or natural humeral head.

A set according to embodiments of the present invention may further include a fifth glenoid component having a fifth joint surface and a fifth bearing surface, and a sixth glenoid component having a sixth joint surface and a sixth bearing surface, wherein each of the fifth and sixth joint surfaces is also configured to cooperate with the prosthetic or natural bone head, wherein each of the fifth and sixth bearing surfaces is also configured for placement against the socket of the joint, wherein the fifth and sixth glenoid components are of a third size larger than the second size, wherein the fifth joint surface is substantially the same as the sixth joint surface, and wherein the fifth and sixth bearing surfaces have different dimensional geometries. According to such embodiments, the first, second, third, fourth, fifth, and sixth glenoid components may be packaged together in a kit, for example within a common box or container.

According to some embodiments of the present invention, the first bearing surface has a first radius of curvature, the second bearing surface has a second radius of curvature that is greater than the first radius of curvature, the third bearing surface has a third radius of curvature, the fourth bearing surface has a fourth radius of curvature that is greater than the third radius of curvature, the fifth bearing surface has a fifth radius of curvature, and the sixth bearing surface has a sixth radius of curvature that is greater than the fifth radius of curvature. The first radius of curvature may be the same as the third radius of curvature but less than the fifth radius of curvature; and/or the second radius of curvature may be the same as the fourth radius of curvature but less than the sixth radius of curvature, according to embodiments of the present invention. In some cases, the fifth radius of curvature is at least 4 mm smaller, or at least 8 mm smaller, than the sixth radius of curvature.

A method for glenoid implantation according to embodiments of the present invention includes evaluating a geometrical shape of a patient's bone socket, selecting a glenoid component from a set of glenoid components, the set of glenoid components including at least two different sizes of glenoid components, with at least two glenoid components of each size having geometrically different bearing surfaces but similar joint surfaces, wherein selecting the glenoid component includes first selecting a size of the at least two different sizes, and then within the selected size selecting a best match of the geometrically different bearing surface with the geometrical shape of the patient's bone socket, and implanting the selected glenoid component in the patient's bone socket.

While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of a glenoid component of a set of glenoid components, according to embodiments of the present invention.

FIG. 2 illustrates a cross-sectional view of the glenoid component of FIG. 1 along plane II of FIG. 1, according to embodiments of the present invention.

FIG. 3 illustrates a set of glenoid components, according to embodiments of the present invention.

While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

A set of glenoid implants according to embodiments of the present invention allows the surgeon to improve the durability of the mechanical cooperation between the implanted glenoid component and the socket of the patient. As such, embodiments of the present invention include a set of glenoid components of a shoulder prosthesis, including glenoid components that each include a body defining, on two of its opposite faces, respectively, a joint surface, intended to cooperate with a humeral head, prosthetic or natural, and a bearing surface bearing against the socket of a shoulder blade, in which set the glenoid components are provided in several different sizes, respectively defined by the dimensions of their respective joint surfaces, the set being characterized in that, in each size, at least two glenoid components are provided with bearing surfaces respectively having different dimensional geometries.

According to one embodiment of the present invention, the surgeon is provided with different sized glenoid components (e.g. at least two different sizes), each of which are available with respective support surfaces provided to be different from each other. In this way, to place a prosthesis in the shoulder of a given patient, the surgeon decides on the size of the glenoid component based on considerations related to the joint to be reestablished with the patient's humeral head, prosthetic or natural, then, given the size that the surgeon has selected, the surgeon chooses the glenoid component with the bearing surface best suited to the patient's glenoid, in its natural actual size or after having surgically prepared it. During surgery, the surgeon strives to keep the greatest possible amount of bone material actually present. Thus, use of embodiments of the present invention is counterintuitive to traditional dogma, according to which the surgeon must perform fairly substantial bone preparation operations for the patient's socket by removing a significant quantity of bone matter from the shoulder blade, to shape, in the patient's shoulder blade, a glenoid cavity adapted to the unique geometry and imposed on the glenoid bearing surface of the component that the surgeon has chosen by its size, e.g., as a function of its joint surface. Conversely, while leaving the surgeon the choice of a size for the glenoid component to be implanted, the glenoid component according to embodiments of the present invention allows the surgeon to take into account the actual state of the patient's glenoid. Furthermore, the bone state of patients' sockets varies within non-negligible proportions, in a manner not correlated to the joint cooperation to be reestablished using a shoulder prosthesis, according to embodiments of the present invention.

According to embodiments of the present invention, the differences between the bearing surfaces of the different glenoid components of a same size can follow the overall geometric size of these surfaces, or, for a given geometric shape, the dimensions of said geometric shape. Of course, the number of sizes present in the set according to embodiments of the invention as well as, within a same size, the number of glenoid components having different respective bearing surfaces, can be as large as desired.

Embodiments of a glenoid component may include a combination of one or more of the following features and/or characteristics, according to embodiments of the present invention:

the dimensional geometries, respectively provided to be different for at least two of the glenoid components having a same size, correspond to a same convex curved surface shape but have as many different curves;

the same convex curved surface shape is substantially spherical, so that said dimensional geometries, respectively provided to be different for at least two of the glenoid components having a same size, correspond to as many different curve radii;

for at least some sizes, the difference between the respective curve radii of the bearing surfaces of said at least two components having a same size is greater than 4 mm, or in some cases greater than 8 mm;

at least one of the curve radii, respectively provided to be different for a same size, has a value that is unchanged at least once when, among the sizes, one goes from a smaller size to a larger size;

at least one of the curve radii, respectively provided to be different for a same size, has a value that increases at least once when, among the sizes, one goes from a smaller size to a larger size;

the dimensional geometries, respectively provided to be different for at least two of the glenoid components having a same size, correspond to as many different geometric shapes selected from the group consisting of: a planar surface, a convex curved surface, a concave curved surface, and a terraced surface;

at least one of said dimensional geometries, respectively provided to be different for at least two of the glenoid components having a same size, is found identically in at least two of the different sizes;

at least one of the glenoid components of the set is provided with at least one anchor element in the socket of the shoulder blade, which protrudes from the bearing surface of said glenoid component;

the bearing surface of each of the glenoid components of the set occupies, outside the anchoring element(s) that may be present, most or all of the corresponding face of the body of the glenoid component.

FIGS. 1 and 2 show a glenoid prosthesis component S2. This component S2 has a primary body S2.1 that has sides, in two spatial dimensions, that are significantly larger than its side in its third spatial dimension, this third side thus corresponding to the thickness of the body. The body S2.1 therefore has two opposite primary faces, connected by a peripheral face embodying the aforementioned thickness of the body.

The body S2.1 can also be a single piece, as shown in FIGS. 1 and 2, or correspond to the fixed assembly of several parts. Furthermore, the body S2.1 is made from one or more materials having a suitable rigidity and strength to, as described in more detail below, both cooperate with a humeral head (not shown) and be securely fixed to the bone socket of a shoulder blade (also not shown in the figures).

One of the two aforementioned primary faces of the body S2.1 defines a joint surface S2.2 shaped to engage with the substantially complementary surface of the aforementioned humeral head. In practice, this humeral head is either natural, e.g. it corresponds to the natural upper epiphysis of the humerus associated with the shoulder blade, or prosthetic, e.g. it is defined by a humeral component of a shoulder prosthesis to which the glenoid component S2 belongs.

The joint surface S2.2 of the body S2.1 is substantially spherical, according to embodiments of the present invention. The corresponding curve radius is denoted R_(S). In the glenoid component shown in FIGS. 1 and 2, this joint surface S2.2 is concave. As a non-illustrated alternative, this joint surface may be convex. The joint surface S2.2 may occupy most of the corresponding face of the body S2.1, or all of said face, as for example illustrated in FIGS. 1 and 2, according to embodiments of the present invention. The other primary face of the body S2.1 is adapted to be securely fastened to the aforementioned socket. To that end, this face includes a bearing surface S2.3 on the socket, from the central region from which an anchoring keel S2.4 for anchoring in the socket extends. According to embodiments of the present invention, outside the anchor keel S2.4, the bearing surface S2.3 advantageously occupies most of the corresponding face of the body S2.1, or all of that face, as for example in the glenoid component illustrated in the figures.

As a non-illustrated alternative, bone anchoring elements other than the keel S2.4 can be provided, to replace or complement said keel. Likewise, such anchor elements can be completely absent, according to embodiments of the present invention. In the glenoid component shown in FIGS. 1 and 2, the bearing surface S2.3 of the body S2.1 is substantially spherical, in other words, it corresponds to a sphere portion for which the curve radius is denoted r_(S2). The surfaces S2.2 and S2.3 may or may not be concentric. In the event they are not concentric, their respective geometric center may or may not be situated in a same median plane of the body S2.1, according to embodiments of the present invention.

According to some embodiments of the invention, the glenoid component S2, which is described in detail above with respect to FIGS. 1 and 2, belongs to a set J of six glenoid components, as shown in FIG. 3. Thus, in addition to the glenoid component S2, which appears in the upper right corner of FIG. 3, the set J comprises five other glenoid components, respectively referenced S1, M1, M2, L1 and L2.

Each of the components S1, M1, M2, L1 and L2 comprises a body S1.1, M1.1, M2.1, L1.1 and L2.1, which is functionally similar to the body S2.1 of the component S2. In particular, each body S1.1, M1.1, M2.1, L1.1, L2.1 has, opposite one another, a joint surface S1.2, M1.2, M2.2, L1.2 and L.2, functionally similar to the joint surface S2.2 of the component S2, and a bearing surface S1.3, M1.3, M2.3, L1.3, L2.3 functionally similar to the bearing surface S2.3 of the component S2.

Component S1 is the same size as component S2, in that their respective joint surfaces S1.2 and S2.2 have substantially the same dimensions. In particular, as shown in FIG. 3, these joint surfaces S1.2 and S2.2 are substantially spherical and have the same curve radius R_(S) according to embodiments of the present invention. The components S1 and S2 thus form, within the set J, a group of size S. According to some embodiments of the present invention, the joint surfaces S1.2 and S2.2 are identical or have curved surfaces which are identical.

On the other hand, component S1 differs from component S2 by the dimensional geometry of its bearing surface S1.3 compared to that of the bearing surface S2.3 of component S2. In other words, the bearing surfaces S1.3 and S2.3 both have the same sphere portion shape, but their respective curve radii r_(S1) and r_(S2) are different: r_(S1) is smaller than r_(S2), according to embodiments of the present invention.

Likewise, components M1 and M2 are the same size and thus form, within the set J, a group of two components, referenced M in FIG. 3. By comparison to components S1 and S2, the joint surfaces M1.2 and M2.2 of components M1 and M2 have substantially the same dimensions, in particular, in the illustrated example, the same curve radius R_(M). The group of size M, containing components M1 and M2, differs from group S, containing components S1 and S2, in that the radius R_(M) has a value greater than that of the radius R_(S), according to embodiments of the present invention.

Furthermore, as for components S1 and S2 in the group of size S, the bearing surfaces M1.3 and M2.3 of components M1 and M2 of the group of size M have different dimensional geometries, e.g. these bearing surfaces are both sphere portions for which the respective radii r_(M1) and r_(M2) are different, the value of the radius r_(M1) being smaller than the value of the radius r_(M2).

The radii r_(S1) and r_(M1) illustrated in FIG. 3 may have the same value and the radii r_(S2) and r_(M2) may also have the same value, according to embodiments of the present invention. In other words, when one goes from the group of size S to the group of size M, the curve radii r_(S1) and r_(M1) of the bearing surfaces S1.3 and M1.3 have an unchanged value, according to embodiments of the present invention. The same may be true for the bearing surfaces S2.3 and M2.3, according to embodiments of the present invention.

Lastly, by transposition of the preceding technical considerations relative to the components S1 and S2 of the group of size S and components M1 and M2 of the group of size M, components L1 and L2 constitute a group of size L, such that:

-   -   the joint surfaces L1.2 and L2.2 of the components L1 and L2         have substantially the same dimensions, in particular the same         curve radius R_(L), which has a value strictly greater than that         of the radius R_(M), according to embodiments of the present         invention, and     -   the bearing surfaces L1.3 and L2.3 are both sphere portions         having respective curve radii r_(L1) and r_(L2), the radius         r_(L1) having a value below that of the radius r_(L2), according         to embodiments of the present invention.

The radius r_(L1) may have a value greater than that of the radius r_(M1), but smaller than that of the radius r_(M2), according to embodiments of the present invention. And the radius r_(L2) may have a value greater than that of the radius r_(M2), according to embodiments of the present invention.

The radii of curvature R_(S), R_(M) and R_(L) may not only increase from size S to size M to size L, but the joint surfaces of the glenoid components of each of the groups with the same size may also differ by an increase in edge-to-edge expanse, for example in the plane of FIG. 3, according to embodiments of the present invention. This difference in the edge-to-edge expanse is optional, however, and is not present in all embodiments of glenoid component sets, according to embodiments of the present invention.

The glenoid components of the set J are broken down into three groups of different sizes: S, M and L, according to embodiments of the present invention. Depending on whether the surgeon chooses one of components S1 and S2 or one of components M1 and M2 or one of components L1 and L2, the joint behavior of the selected glenoid component, relative to the humeral head of the patient, is fixed. However, for each of sizes S, M and L, the surgeon's choice of one of the two available components allows him to account for the actual state of the patient's socket: in particular, the surgeon will choose the one of the two available components for which the curvature of the bearing surface is best suited to the actual geometric configuration of the socket, in particular to limit the extent of the prior preparation of the socket, thereby limiting the loss of bone substance, while also optimizing the extent of the bearing interface produced between the socket and the component selected during implantation thereof, according to embodiments of the present invention.

The difference between the radii of curvature of the bearing surfaces of the components of a same size is, for example, greater than or equal to 4 mm, according to embodiments of the present invention. According to other embodiments of the present invention, the difference between the radii of curvature of the bearing surfaces of the components of a same size is greater than or equal to 8 mm. Thus, one non-limiting example of numerical quantification related to the embodiment of the set J shown in FIG. 3 is as follows:

r_(S1)=34 mm,

r_(S2)=38 mm,

r_(M1)=34 mm,

r_(M2)=38 mm,

r_(L1)=36 mm, and

r_(L2)=44 mm.

Various arrangements and alternatives may be utilized for the set of glenoid components J. As one example, at least one of the three groups of different sizes S, M and L may include more than two glenoid components, the respective bearing surfaces of the at least three glenoid components respectively having different dimensional geometries. Likewise, the set can be provided with only two groups of different sizes, for example S and M, or S and L or M and L, according to embodiments of the present invention. The set can also include more than three groups of different sizes, according to embodiments of the present invention.

According to one alternative embodiment of the present invention (not shown), the difference of dimensional geometries between the bearing surfaces of the glenoid components of a same size does not correspond to a dimensional difference of a same geometric shape, as is the case for the set J, but this difference is based on the presence of as many different geometric shapes as there are glenoid components present in the group of a same size. In other words, within each group of a same size, one of the glenoid components has, for example, a planar bearing surface while the other glenoid component has a bearing surface with a convex curve, according to embodiments of the present invention. More generally, the respective bearing surfaces of the at least two glenoid components present in the aforementioned group then have different geometric shapes that are chosen from among at least a planar surface, a convex curved surface, a concave curved surface and a terraced surface, according to embodiments of the present invention.

Optionally, rather than being continuously smooth as in the figures, the bearing surfaces of the components of the set J can have a roughness, or even macro-aspects, such as grooves, pyramidal spurs, conical cells, and the like, to strengthen the immobilization of the component in the socket.

Although embodiments of the present invention are described with respect to shoulder joints, one of ordinary skill in the art, based on the disclosure provided herein, will recognize that similar sets of glenoid components may be used for other joints, for example hip joints, according to embodiments of the present invention.

Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof. 

What is claimed is:
 1. A set of glenoid components for use in a shoulder joint, the set comprising: a first glenoid component comprising a first primary body and a first anchoring element, the first primary body comprising a first joint surface configured to cooperate with a prosthetic or natural bone head and a first bearing surface configured for placement against the patient's shoulder blade socket, the first anchoring element extending from the first bearing surface; and a second glenoid component comprising a second primary body and a second anchoring element, the second primary body comprising a second joint surface configured to cooperate with the prosthetic or natural bone head and a second bearing surface configured for placement against the patient's shoulder blade socket, the second anchoring element extending from the second bearing surface; wherein a difference between a radius of curvature of the first bearing surface and a radius of curvature of the second bearing surface is at least 4 mm.
 2. The set of claim 1, wherein both the first and second bearing surfaces comprise convex curved surfaces.
 3. The set of claim 1, wherein both the first and second joint surfaces comprise concave curved surfaces.
 4. The set of claim 1, wherein both the first and second bearing surfaces comprise smooth surfaces.
 5. The set of claim 1, wherein the first anchoring element and the second anchoring element are keels.
 6. The set of claim 1, wherein the first and second glenoid components are of a first size, the set further comprising: a third glenoid component comprising a third joint surface and a third bearing surface; and a fourth glenoid component comprising a fourth joint surface and a fourth bearing surface, wherein each of the third and fourth joint surfaces is configured to cooperate with the prosthetic or natural humeral head, and each of the third and fourth bearing surfaces is configured for placement against the shoulder blade socket of the shoulder joint, the third and fourth glenoid components are of a second size larger than the first size, a radius of curvature of the third joint surface is the same as a radius of curvature of the fourth joint surface, a radius of curvature of the third bearing surface is different than a radius of curvature of the fourth bearing surface.
 7. The set of claim 6, wherein a radius of curvature of the first bearing surface is the same as a radius of curvature of the third bearing surface.
 8. The set of claim 7, wherein a radius of curvature of the second bearing surface is the same as a radius of curvature of the fourth bearing surface.
 9. The set of claim 6, wherein the radius of curvature of each of the third and fourth joint surfaces is greater than the radius of curvature of each of the first and second joint surfaces.
 10. The set of claim 6, wherein the joint surfaces of the third and fourth glenoid components have an increased edge-to-edge expanse compared to the joint surfaces of the first and second glenoid components.
 11. The set of claim 6, further comprising: a fifth glenoid component comprising a fifth joint surface and a fifth bearing surface; and a sixth glenoid component comprising a sixth joint surface and a sixth bearing surface, wherein each of the fifth and sixth joint surfaces is configured to cooperate with the prosthetic or natural humeral head, each of the fifth and sixth bearing surfaces is configured for placement against the shoulder blade socket of the shoulder joint, the fifth and sixth glenoid components are of a third size larger than the second size, a radius of curvature of the fifth joint surface is the same as a radius of curvature of the sixth joint surface, and a radius of curvature of the fifth bearing surface is different than a radius of curvature of the sixth bearing surface.
 12. The set of claim 11, wherein a radius of curvature of each of the fifth and sixth joint surfaces is greater than the radius of curvature of each of the third and fourth joint surfaces.
 13. The set of claim 11, wherein the joint surfaces of the fifth and sixth glenoid components have an increased edge-to-edge expanse compared to the joint surfaces of the third and fourth glenoid components.
 14. The set of claim 11, wherein a radius of curvature of the fifth bearing surface is greater than the radius of curvature of the third bearing surface but smaller than the radius of curvature of the fourth bearing surface.
 15. The set of claim 14, wherein a radius of curvature of the sixth bearing surface is greater than the radius of curvature of the third bearing surface and the fourth bearing surface.
 16. The set of claim 1, wherein the difference between the radius of curvature of the first bearing surface and the radius of curvature of the second bearing surface is at least 8 mm. 